Television transmitter



oct. 27, `1942.

G. L. FREDENDALL TELEVISION TRANSMITTER Filed o ct. 24, 1940 lhwentor Lik-lengua Patented Oef.. 27, 1942 UNITED' STATES( Pari-:N

r OFFICE- '2.299.891 mavrsIoN mmsm'rrm Gersen r.. Fred to Radio Corporation of Delaware Westmont, N. J., assigner ofYA'mex-lca, a corporation application october 24,1940, seran Ne. 362,494

' s claims. ici. 17a-7.2i

My invention relates to piemre transmitters and particularly to transmitters of the type employing cathode ray pickup or transmitter tubes. Specifically, the invention relates to an improvement in the transmitter described and claimed in application Serial No. 362,547, iiled onv the same'day as the present applicatiomin the name of Alda V. Bedford, entitled Television transmitter, and assigned to the Radio Corporation of America.

One ofthe major problems in television is that of obtaining a good picture signal Ato noise ratio without using too brilliant-illumination on the subject. being televisedor without opening the the camera so wide that the lens aperture of v depth of focus is too shallow.I At times the problem is simply that of obtaining a usable picture signal under the lighting conditions available.

At 'one time it was the practice to make the resistance of the output resistor of' a cathode ray pickup tube low enough to give the output circuit of thistube a ilat frequency-response characteristic over the entire picture frequency range.

More recently the output resistor has been given a much higher value than indicated above whereby the picture signal components are held up to a good signal strength for the` lower frequencycomponents (up to about 100 kilocycles,

for example). Generally the output resistor has a resistance of from 50,000 ohms to 200,000 ohms. I'he response characteristic is not dat, however, and falls off for the higher frequencies, this falling off of the characteristic being compensatedA circuit just described for in a later stage. The is usually referred to as the high peaker` circuit and isl covered by Bedford Patent 2,151,072, issued March 21, 1939, and assigned to the Radio Corporation of America.

An object of the present invention is to provide an improved transmitter circuit wherein the signal-to-noise ratio of a picture signal is improvedby holding up or accentuating the picture signal amplitude in the region of the higher Vfrequency components and later compensating for this accentuation.

Stated more broadly. an object of the invention is to provide anA improved circuit for increasing the usable signal output of a -picture transmitter tube.

A further object of my invention is to provide improved picture transmitter apparatus in which the signal-to-noise ratio of its output is improved and in whichra simple and easily adjusted correcting or compensating network is employed.

In practicing the` invention, an 4inductance coil 'is connected between the output electrode of the cathode ray transmitter tube andthe control grid of the iirst amplifier whereby the outcapacities are about 10 micro-microfarads each and where the inductance coil is adjusted for a cut-oh' frequency ofabout 4.5 megacycles. Because of this, the separation of two capacities in the pickup tube output'circuit by means of an inductance coil formerly was considered undesirable.

In my improved transmitter, the conventional theory that major reiiections at the end of a iiltershould notbe allowed is discarded as taughtv by the above-mentioned Bedford copending application. Instead of terminating the illter section in its surge impedance of 3,000 ohms, for example, it is terminated in a very high resistance such as 100,000 ohms. This results in a tremendous peak in the picture signal ina certain region of the higher frequency components, the location of this peak depending upon the inductance value of the coil. y

After a suitable amount of amplication, the picture signal is passed through a compensating circuit which has the proper characteristic to make the, overall frequency response characteristic of the pickup tube and associated amplier vand compensating circuit substantially flat.`

This means, of course, that the compensating circuit i'svso designed that the picture signal components in the region of the above-mentioned peak vare ampliiied less than the picture signal components of less amplitude. The result is that the noise, also, in the region of picture signal peak is amplied less whereby the overall signal-to-noise ratio is improved. 'Ihe compensating circuit, in addition to providingamplitude correction, corrects for undesirable phase shift which is introduced by the above-mentionedinductance coil. Y

`I have improved on the transmitter apparatus which is being claimedV in `the above-identied Bedford application by providing a correcting or response of the circuit of Fig. 1 when my cor-v recting network is omitted,

Figure 3a is a curve showing the square wave response of the circuit of Fig. 1 when my cor,`

recting network .is included, and

Figure 4 illustrates another embodiment of my invention.

4Referring to Fig. 1, the transmitter circuit comprises a cathode ray transmitter or pick-up tube I0 consisting of an evacuated envelope having therein an electron gun I I for producing a,

beam of electrons, a second anode I2 and a mosaic I3 of light sensitive elements. In this particular type of pick-up tube alight image of the picture to be transmitted is projected upon the mosaic I3 by means of a suitable lens system, not shown. This causes an electrical image of the picture to be formed on the mosaic whereby picture signals representative of the conditions of light and shade in the picture are produced as the mosaic is scanned by the electron beam. 'Ihe beam is caused to scan the mosaic by means .of defiecting coils or plates, not shown. It will be understood that theinvention is not limited to the specific type of pick-up tube I0, this type of tube being selected merely for the purpose of giving a specific example.

As stated in Bedfords above-mentioned high peaker patent, and as well known in the art at the present time, the mosaic I3 may be constructed in various ways. In all cases, however, it will be found that there is a substantial amount of capacity between the output electrode of the pick-up tube and ground, and that this is one of the limiting factors in obtaining a picture signal output of substantial amplitude at the higher frequency components of the picture signal. As explained in the said Bedford patent, this difficulty can be overcome to a certain extent by making the output resistance of the cathode ray tube much higher than the value required for a flat over-all response and compensating at a later point in-the circuit for the resulting falling of! of the signal amplitude at the higher frequency components.

In the transmitter of Fig. 1, in addition tov giving the output resistor, shown at I6, a high resistance value as taught in the Bedford high peaker patent, an inductance coil L is connected between the output terminal of the cathode ray tube I0 and the control grid IIl of the first amplifier tube I8. whereby the output capacity of the cathode ray tube, indicated at CI, and the input capacity of the firstamplier tube,

indicated at C2, are separated. It will be levident that now, instead of the sum of the capacities C I and C2 appearing between the output terminal of the cathode ray tube and ground', the cathode ray tube feeds into a section of a low pass filter CI-L-C2. Y v

In terminating this filter section CI-L-C2, however, conventional nlter theory is not followed since lter theory would require that the output resistor I6 (which terminates the lter section) have a resistance very much lower than the resistance actually employed. As previously indicated, filter theory would require that the filter section be terminated in a resistance of about 3,000 ohms, whereas, in practicing my invention. the filter section is terminated in a resistance of 100,000 or 200,000 ohms,for example.

As a result of the high resistance filter termination, the frequency response curve, taken at the control grid I1 of the first amplifier tube, is represented by the curve BI in Fig. 2. It will be seen that this curve has a very high peak in the region of the high frequencycomponents of the picture signal as a result of the high resistance termination of the filter section. It will be noted that curve BI also has a large amplitude in the region Yof the lower frequency components of the picture signal as a result of the benefit derived from the high resistance of the. output resistor I6 as described in the Bedford high peaker patent.

Perhaps it should be mentioned that the frequency response curves in the present application and those shown in the Bedford high peaker Patent No. 2,151,072 are drawn to scales which are so unlike that at first glance corresponding curves in the two cases do not look alike. In the high peaker patent, the curves are drawn up to one' megacycle only and they are drawn to a logarithmic scale which stretches out" the low frequency end of theV scale. On the other hand, vthe frequency scale for the curves of'Fig. 2 in the present application is linear and goes out to 5 megacycles.

In actual practice, it usually is desirable to accentuate the very low frequency components, components of the order'of 60 or 100 cycles per second, for example, by.means of a condenser 2l connected in series with the output resistor I6 `and shunted by a resistor 22. The desired negative bias for the amplifier tube I8 may be provided by a biasing battery 23. In the illustration lci Fig. 1, the condenser 2| and resistor 22 are shown short circuited by means of a switch 24 to make them ineffective. The curves in Fig. 2 are, drawn for the condition of operation where the switch 24 is closed.

The use of the circuit 2I-22 for accentuating the very low frequency components is described in Bedford Patent No. 2,200,073, issued May '1,

4194.0. Here again, the frequency scale for the curves in the patent is logarithmic, whereby the curve in the region of the lower frequencies is stretched out. Since this accentuation of the signal occurs only in the general region of cycles it is apparent that it could not very well be shown in Fig. 2 where the frequency scale is linear and covers 5 megacycles.

Still referring to Fig. 1, the amplifier tube I8 preferably is provided with the usual inductance coil 26 for holding up the high frequency response. The output of this amplifier is passed through one or more amplifier` stages, suchv as the one including amplier tube 21, and supplied to my improved correcting or compensating network indicated at 28. 'I'his network, which will be described -in detail hereinafter, is designed to have the characteristic B2 of Fig. 2. y

The operation of the circuit embodying my invention will now be more fully explained with reference to the curves in Fig. 2. The curve AI. which at the lower `frequencies coincides with the curve BI, represents the frequency response characteristic of the circuit of Fig. 1 with the resistor 32, and a coupling condenser 33.

een r. omitted. It win be'evident that the use of the high resistance output resistor Il has held up the picture signal output in the region below one megacycle. vFollowing the teachings of Bedford Patent 2,151,072, a il'at overall response,

such as shown by the curve A3, would be obtained by means of a later compensating or high peak er" stage havingthe frequencyresponse characteristic shown by the curve A2. i

With' the coil L included in the circuit, as illustrated in Fig. 1, the frequency response measured at the control grid I1 of the first ampliiler I tube is that shown at the curve BI, there being a high peak in this curve in the region of the higher frequency components of the picture signal, this peaking being located at about 4 megacycles in the particular example illustrated.

As previously stated, the correcting or compensating network 28 has the frequency response characteristic shown at B2. The frequency response characteristic B2 is made 'such that the overall frequency response is flat, this flat over- .than can be Obtained with allfrequency response being represented .by the curve A3 previously referred to. It will be noted that the flat overall frequency response curve A3 is obtained if the ordinates of curves BI and B2 and multiplied together rather than added tc-' gethersince the overall gain of two stages in cascade is obtained by multiplying rather than vby adding the gains of the individual stages.

Referring now to the design of the correcting network 28,1t comprises the network R'i-Cl-RS connected across the output circuit of ampliner 21, the resistance of R1 being sfohlgh that the network puts substantially no load on the amplifler 21. The network R1-C1--R5 is one of the types of high peaker networks described in Bedford Patent 2,151,072 and, by itself, has essentially the frequency response characteristic A2 (Fig. 2).

Theresistance of resistor R5 is .low compared with' that of resistor R1 whereby the amount o f current owing. through they condenser C1 and .ample illustrated. same is for the ampliner stage II.

It may be found desirable in some instances to connect a condenser Cil across the seriesresonant circuit LI2-CI2-Ri2 by means of a switch I6. iialculations and tests show that this circuit will provide more perfect correction condenser Cil omitted. A Some of the circuit constants'for the specific apparatus shown in Fig. 1 are given below, by

way of example, the capacity and inductance being given in micro-microfarads and microhenrys:

It has been foundthat the circuit of Fig. 1

With switch I may b'e adjusted very easily to produce a picture 4sponse of the cathode ray output circuit, this through resistor R5 increases substantially as the signal frequency increases, this resulting in the rising characteristic A2 and in the rising portions of the curve B2.

Thej'dip in the curve B2, occurring between three and four megacycles in the example shown,

is produced by a series-resonant circuit comprising inductance coil LI2. a condenser Ci2 and a resistor RI2 connected across the resistor R5'.v

The circuit LI2-Ci2--RI2 is series-resonant at lthe same frequency at which the `resonant peak in the curve BI appears, as will be evident from a comparison of curves Bl and B2. 'I'he desired amount of damping in the circuit LI2CI2R|2 for properly shaping the curve B2 is introduced -by adjusting the resistance value of RI2.

It has been found that the above-described network 28 may be adjusted to compensate for the characteristic of the preceding part of the circuit. so accurately that a picture of excellent quality is obtained. This means, of course, that thenetwork compensates for both the amplitude and the phase distortion that has been introduced by the combination L-C|'C2.

'The output circuit of the amplifier 21,'across which the network .28 is connected, preferably? includes the usual inductance coil 3l for holding up the amplifier high frequency response, a plate specifically, the ampliiier stage 21 (without the compensating network 28) is designed to have a flat frequency response characteristicin the ex of good quality. In adjusting the circuit, the

series-resonant frequency of the circuit Li2-Cl2 is made equal to the frequency of the peak rebeing 3.8 niegacycles in the example illustrated. 'I'he ratio of LI2 to C|2 is varied until the best compensation is obtained. Usually the adjustments may be made by observing a test pattern. The values of R1, C1 and R5 are determined by causing this portion of the circuit to compensate for the cathode ray tube output circuit when the coil L in that circuit is short-circuited, and when the. series-resonant circuit LI2-Cl2 is opened. As a final adjustment with all elements included in the circuit, the resistor RI2 is adjusted sol that the overall amplitude response of the complete circuit illustrated in Fig. 1 is flat. The characteristic of the circuit of Fig. l with and without the correcting network 2011s further illustrated by Figs. 3 and 3a. Fig. 3 shows the square wave response of the circuit with the coil Lbut without the correcting network, while Fig. 3a shows the square wave response of the circuit after the undesirable transient' response More 4 introduced by the coil L has been compensated for by means of the correcting network.

The great improvement that may be obtained in the signal-to-noise ratio by practicing my invention will be seen more clearly from the following considerations. The curves A2 and B2, respectively, .indicate the actual amplification following the grid I1 of thei'irs't amplifier tube that is necessary in order to obtain the same picture signal output for the case where the inductance coil L is omittedk as for the case where it is included in the circuit. Since most of the noise in the system generally originates in the first amplifier tube i8 (assuming the case where the output resistor I6 has large resistance), the relative noise obtained with my invention as compared with that obtained withoutmy invention is shown by a comparison of the ordinates v peak in the curve Bi;

of curve B2 and A2, respectively. Obviously, my invention causes a substantial reduction in noise in the picture signal since the curve B2 is much lower .in amplitude than the curve A2 over a large portion of the upper frequency range.

The position of the peak of the curve BI in the high frequency region may be shifted either toward the high end or toward thevlow end of the frequency band by changing the value of the inductance coil L. It may be preferable to have this peak located at a slightly dierent frequency than shown in Fig. 2 if maximum noise reduction is to be obtained.

It is usually desirable to operate the circuit of Fig. 1 with the switch 24 open whereby the previously described accentuation of the very low frequency components is obtained. It will be understood that, when so operated, there is provided at some later point in the circuit a complementary attenuating means such as a small coupling condenser, as described in Bedford 'Patent No. 2,200,073. v

In Fig. 4 there is shown an embodiment of my invention in which a parallel resonant circuit is utilized in place of the series resonant circuit LI2-CI2 of Fig. 1. Like parts'in Figs. 1 and 4 are-indicated by the same reference characters.

Vsistor to increase with increase in signal frequency, and a network connected across said last named resistor for decreasing the output voltage of the correcting network at the frequency of said resonant peak.

2. Picture transmitting apparatus comprising an electric discharge device for converting pictures into electrical signals, said device having an output terminal which has unavoidable capacity to ground, an amplifier tube having a conconnected in cascade with said amplifier tube and designed to correct for amplitude and phase distortion introduced by said filter section, said correcting network comprising a resistor from across which the-output signal is taken, means At ZI there is indicated an impedance network,

corresponding to RT--C'l of Fig. 1, which causes the current flowing through R5 to increase with increase in signal frequency. The network Z2 is connected in series with the lead supplying signal from R5 to the succeeding amplifier (not shown) for producing the desi-red dip inthe curve B2. It comprises a parallel resonant circuit LIl-CM tuned to resonateat the frequency of the resonant The resistor RII may be the grid leak of the above-mentioned succeeding amplifier.

From the foregoing description, .it will be apparent that various other modifications may be made in my invention without departing from the spirit and-scope thereof.

I claim as my invention:

1. Electrical apparatus comprising a signal output device, said device having an output terminal which has unavoidable capacity to ground, an amplier tube having a control electrode which 4has unavoidable capacity to ground. aninductance coil connecting said output terminal to said control electrode whereby said coil and said capacities form a filter section. a termination for said filter section comprisinga resistor having such high resistance that said section is terminated in an impedance many times its Surge impedance whereby the picture signal output appearing at said control electrode has a resonant peak in'the region of its high frequency components. and a correcting network connected in cascade Vwith said amplifier tube and designed 'to correct foramplitude land phase distortion introduced by said filter section. said correcting network comprising a resistor from .across which the output signal is taken, means for causing-the signal current now through said lastnamed're-4 for causing the signal current ow. through said last named resistor to increase with increase in signal frequency, and a resonant circuit connected to said last named resistor and tuned to resonance substantially at the frequency of said resonant peak to reduce the amplitude of the signal appearing across said last named resistor atv saidfrequency.

'3. The invention according to claim 2 wherein the last named means comprises a resistor of comparatively high resistance in-series with the last named resistor, and a condenser connected in parallel relation to said high resistance resistor.

4. 'I'he invention according to claim 2 wherein the said resonant circuit is a series resonant circuit connected in parallel with the last named resistor.

5. The invention according to claim 2 wherein' the said resonant circuit is a parallel resonant circuit connected in series with an output lead4 from said last named resistor.

6. In picture transmitting apparatus, a correcting network for a picture transmitter tube output circuit which is in the form of a low pass filter section terminated by a resistor having much higher resistance than the surge impedance of said filter section, said correcting network being connected in cascade with said output circuit and comprising a resistor and a condenser connected in parallel with each other, a second resistor connected in series with said parallel combination, a series resonant circuit connected series combination of flrst resistor and second resistor, and means for taking the corrected sigv nal from across said second resistor.

GORDON L. FREDENDALL;v 

